1. Field of the Invention
This invention relates generally to the copolymerization of ethylene with 1-olefins and, more specifically, this invention relates to the high temperature and high pressure copolymerization of ethylene and 1-olefins to produce low density ethylene copolymers.
2. Description of the Prior Art
The copolymerization of ethylene with 1-olefins under high pressure, high temperature conditions to produce low density polyethylene copolymer resins is well known. Such systems generally utilize coordination metal catalysts (such as so-called Ziegler catalysts, for example) and 1-olefin comonomers such as propylene, 1-butene, 1-hexene, etc. to produce resins having densities within the range of about 0.92 g/cc to about 0.94 g/cc.
The density of an ethylene copolymer is inversely related to the degree of incorporation in the polymer chain of the 1-olefin comonomer. Since the reactivities of such comonomers are relatively low compared to the reactivity of ethylene, prior practice has been to utilize a relatively high concentration of comonomer in the reaction mixture with ethylene, in order to produce low density products.
Unfortunately, the use of relatively high concentrations of 1-olefin comonomers results in an undesirable increase in the dew point of the reaction mixture, which in turn results in undesirable condensation of liquid from the reaction mixture, contributing to possible equipment damage and unsafe operating conditions in low pressure recycle compression systems of conventional low density polymer plants. Also, the use of relatively high concentrations of 1-olefin comonomers limits the lowest achievable product density, and prevents the use of higher boiling point comonomers in copolymerization.
One solution to this problem has been to provide facilities to condense and separate 1-olefin comonomers from normally gaseous recycle streams, followed by injection of the liquid comonomer stream into a recycle stream at a relatively high pressure. However, this requires expensive equipment and results in a substantial increase in the energy cost per pound of product.
Another problem associated with the use of relatively high levels of 1-olefin comonomers is the presence of unreacted high boiling point comonomers in the molten polyolefin product stream. The presence of unreacted monomer at high concentrations can result in polymer foaming and problems in feeding the product extruder. Removal of unreacted comonomer from the product stream may require expensive vacuum separation equipment.